This invention relates to thermoplastic polyolefin (TPO) incorporating polypropylene reinforced with delaminated or exfoliated cation-exchanging multi-layered silicates.
In their natural state, the layers of cation-exchanging multi-layered silicates, such as montmorillonite, are held together by ionic bonds to the exchangable cations. As discussed by Kawasumi et al. in Macromolecules, 1997, 6333–6338, when such silicates are blended with softened or melted polypropylene, the resulting shear forces are not sufficient to delaminate or exfoliate the silicate layers even when the cation is a quaternary ammonium ion because polypropylene is a relatively non-polar material.
Usuki et al., U.S. Pat. No. 5,973,053, solved this problem using two related approaches. The first approach (also described by Kawasumi et al.) was to blend a quaternary ammonium exchanged multi-layered silicate with a maleic anhydride modified polypropylene oligomer and then add an unmodified polypropylene polymer. The maleic anhydride modified polypropylene oligomer had sufficient polarity to exfoliate the silicate under the shear conditions of the blending process.
The second approach of Usuki et al. was to blend a quaternary ammonium exchanged multi-layered silicate with a maleic anhydride modified polypropylene polymer. The maleic anhydride modified polypropylene polymer had sufficient polarity to exfoliate the silicate under the shear conditions of the blending process.
Usuki et al. pointed out that when a maleic anhydride modified polypropylene oligmer was not used, then the average molecular weight of the maleic anhydride modified polypropylene polymer should be limited to about 100,000.
Thermoplastic olefin (TPO) is a mechanical blend of a polyolefin (such as polypropylene) and a thermoplastic elastomer (such as EPDM or ultra low density polyethylene). The use of polypropylene based TPO articles at a low temperature is limited because polypropylene based TPO has relatively poor low temperature impact toughness.